1. Field of the Invention
The present invention relates generally to a Hybrid Electric Vehicle (HEV), and specifically to a system and method of allowing an HEV to operate without damage after the HEV engine cooling system has been compromised. This is accomplished by using both the traction motor and the engine to provide power approximating the power output of the engine while operating on all cylinders, while in fact operating the engine with a reduced number of cylinders.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions from automobiles and other vehicles powered by Internal Combustion Engines (ICES) is well known. Vehicles powered by electric motors attempt to address these needs. Unfortunately, electric vehicles have limited range and power capabilities. Further, electric vehicles need substantial time to recharge their batteries. An alternative solution is to combine a smaller ICE with an electric traction motor into one vehicle. Such vehicles are typically called Hybrid Electric Vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV is described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set. Other, more useful, configurations have developed. For example, a Series Hybrid Electric Vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A Parallel Hybrid Electrical Vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that together provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A Parallel/Series Hybrid Electric Vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is typically known as a “powersplit” configuration. In the PSHEV, the ICE is mechanically coupled to two electric motors in a planetary gear set transaxle. A first electric motor, the generator, is connected to a sun gear, and the ICE is connected to a carrier. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in the transaxle. Engine torque powers the generator to charge the battery and the resultant torque at the ring gear contributes to the wheel (output shaft) torque. The traction motor is also used to contribute wheel torque and to recover braking energy to charge the battery if a regenerative braking system is used. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuously variable speed transmission effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicle by using the generator to control engine speed.
The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drivability. Nevertheless, new ways must be developed to optimize the HEV's potential benefits. One such area of development is in the development of advanced control systems allowing the HEV to continue operation even after an engine coolant system malfunction. It is generally known that malfunctions of engine cooling systems can cause engine damage from the excessive overheating. Such malfunctions often involve loss of coolant. Coolant loss can be sudden due to a leak in the cooling system. Alternatively, overheating malfunctions without coolant loss can occur if the coolant circulation system malfunctions such as a failure of a water pump.
Methods of allowing an engine to continue to operate without damage after coolant system failure are known in the prior art and known as so-called “fail safe cooling.” One such prior art method alternates fueling and firing cutoffs to the engine cylinders. For example, U.S. Pat. No. 5,555,871 to Gopp, et al., describes an engine cylinder head temperature sensor and the control system. When the cylinder head temperature exceeds a threshold, the control system deactivates one or more of the engine's cylinders. The control system rotates the deactivation of the cylinder's spark so that no cylinder is constantly fired. While deactivated, fresh air is drawn through the cylinders and cools the engine. Gopp, which is assigned to the assignee of the present invention, has no applicability to operation of an HEV with both an engine and a rotating electrical machine. Rather, Gopp is concerned solely with a conventional engine/transmission configuration.
The prior art also describes alternating fuel flow to deactivate a cylinder bank when a temperature threshold is passed. In this mode, the air fuel mixture of the activated cylinder bank is adjusted to limit vehicle speed and extend the operating time. See generally, U.S. Pat. No. 4,473,045 to Bolander, et al.
Various other overtemperature control systems exist in the prior art. In U.S. Pat. No. 5,094,192 to Seiffert, et al., ignition slows in response to coolant pump failure. This limits the load and speed of the engine.
Other methods reduce engine heat when the coolant system fails to trigger a water pump or cooling fans to cool the engine compartment. U.S. Pat. No. 4,977,862 to Aihara, et al. Similarly, U.S. Pat. No. 5,065,705 to Fujimoto, et al., describes a system based on engine speed that reduces engine power output if it predicts the overheating of the engine.
Although prior art control systems are useful when applied to conventional ICE vehicles, the HEV can utilize additional methods to reduce engine damage when its coolant system fails. For example, it can limit or even shut down engine operation and provide torque through its traction motor. Japanese published application JP 40604818A to Kitada discloses this type of system, in which the HEV engine is shut down if the engine temperature exceeds a predetermined threshold. Unfortunately, such operation severely impairs the operational functionality of the HEV because once the traction battery is depleted, the vehicle will slow to a halt.
The HEV provides other design challenges which remain unsolved in the prior art. For example, prior art systems typically apply to a large ICE with six or more cylinders. In smaller ICES (four cylinder or less) such as those found in an HEV, engine power output, and noise vibration, and harshness (NVH) would be unacceptable using the prior art control systems.